An ultra high-speed transmission system more than 1 Gbit/sec using optical communication techniques has been rapidly developed. An external modulation manner for realizing high-speed responsibility and long distance transmission, for example, a semiconductor optical modulator, has been also developed. Furthermore, an optical semiconductor element integrating a semiconductor laser and optical modulator has been researched.
Such an optical semiconductor element may be effectively fabricated by using the selective growth technique by the MOVPE method. For example, Japanese patent application laid-open No.4-303982 discloses a selective MOVPE method in which the etching for a substrate and a growing layer is not used. Conventionally, when the waveguide width of a growth is controlled by mesa-etching, the mesa width can not be sufficiently controlled by the etching, thereby causing a dispersion in the wafer. On the contrary, in the selective MOVPE method, the mesa width can be easily controlled by changing the width of a mask for blocking the growth and the damage to a substrate and growing layer due to the etching can be avoided to improve the uniformity of an element.
However, in Japanese patent application laid-open No.4-303982, where a device portion is provided on a burying layer grown by the selective MOVPE method from opening regions at both sides of a double-heterostructure (DH), the film thickness of the regions outside the device portion is greater than that of the device portion. This is caused by the fact that the burying layer on the regions outside the device portion is grown from a mesa portion already formed.
Therefore, it is difficult for the glass mask used in the process (the contact exposure technique) of forming electrodes on the device portion to be contacted with the device portion since the outside regions first contact the glass mask. As a result, a mask pattern is not securely transferred to the device portion, thereby failing to remove a coated resist in the form of a stripe, i.e., resulting in "removal failure".
To overcome the above problem, the film thickness of the outside regions should be thinner than that of the device portion to closely contact the glass mask with the device portion. For example, Japanese patent application laid-open No.01-007586 discloses a method for making an optical semiconductor element in which a difference in film thickness is provided before the growth of a burying layer. In this method, a double channel planar burying heterostructure (DC-PBH structure) laser is fabricated only by the MOVPE method. Namely, when the method in Japanese patent application laid-open No.01-007586 is combined with the above problem in Japanese patent application laid-open No.4-303982, the regions outside the device portion will be suitably etched off before the growth of the burying layer.
However, since in such a combined method the growing layer has to be etched off, the advantage of the selective MOVPE technique that a substrate is not damaged by etching and the product yield is thereby improved, is substantially lost. Furthermore, in the combined method, the number of process steps for making an optical semiconductor element is undesirably increased.